In recent years, there is an increasing demand for using a high-strength steel plate for a vehicle body, a component, and so on in an automobile field. This is because weight saving of a vehicle body is necessary for achieving a fuel efficiency and a carbon dioxide gas (CO2) emission reduction and because collision safety should be improved. On the other hand, spot welding is mainly used in processes such as assembling of a vehicle body, mounting of a component, and the like. However, if a high-strength steel plate, in particular a high-strength steel plate whose tensile strength is high, is spot welded, following problems occur.
As characteristics important in a joint which has been spot welded (hereinafter, referred to as a spot welded joint), a tensile strength and a fatigue strength can be cited, and which is important in particular is the tensile strength. As the tensile strengths of the spot welded joint, there are recited a tensile shear strength (TSS) measured by applying a tensile load in a shear direction and a cross tensile strength (CTS) measured by applying a tensile load in a peel direction. Measuring methods for the tensile shear strength and the cross tensile strength are defined in JIS Z 3136 and JIS Z 3137, for example.
In general, if a value of hardness of a nugget is high as well as a value of toughness is also high and segregation in the nugget is suppressed, a fracture appearance in a tensile test is good (a fracture occurs in a periphery of the nugget), a sufficiently high tensile strength can be obtained, and fluctuation thereof is also small. On the other hand, if the value of hardness of the nugget is high but the value of toughness is low, or if the segregation in the nugget is prominent, a peel fracture (a fracture in the nugget) or a partial plug fracture (a partial fracture in the nugget) occurs in the tensile test and a good fracture appearance cannot be obtained. In such a case, a tensile strength is reduced significantly and fluctuation thereof becomes large. The reduction and the fluctuation of tensile strength are exhibited particularly in the cross tensile strength. Further, in spot welding of a high-strength steel plate, a crack or a defect sometimes occurs in a nugget, and in such a case a reduction or fluctuation of joint strength becomes prominent. Thus, in a spot welded portion of a high-strength steel plate, improvement of the fracture appearance and accompanying betterment of the cross tensile strength as well as reduction of fluctuation thereof are required.
It should be noted that a defect or a crack in a nugget is considered to be caused by rapid contraction of the nugget after finishing of energization. In other words, in spot welding, parts of high-strength steel plates melt during energization between two welding electrodes and solidify mainly by heat removal via the welding electrodes after finishing of energization. Since the welding electrodes are water-cooled, rapid contraction sometimes occurs in a thickness direction of the high-strength steel plates in solidification of the molten parts. Thus, as illustrated in FIG. 1A, a contraction defect 105 sometimes occurs in a center of a nugget 103 formed between high-strength steel plates 101A and 101B. Further, the nugget 103 sometimes contracts also in a lateral direction, and the high-strength steel plates 101A and 101B whose temperatures have not risen cannot follow that contraction, and as illustrated in FIG. 1B, the nugget 103 being given a tensile stress from the high-strength steel plates 101A and 101B, a longitudinal crack sometimes occurs in the nugget 103.
As described above, since a good fracture appearance cannot be obtained if a toughness of a nugget is low or segregation in a nugget is prominent, reduction of tensile strength (joint strength) of a spot welded joint and fluctuation of welding strength occur. On the other hand, insufficiency of toughness and prominent segregation are apt to occur when a high-strength steel plate with a high carbon equivalent is spot welded. Methods whose objects are to solve such a problem are described in Non-patent Literature 1 and Patent Literature 1. In these methods, tempering energization is performed after a predetermined time passes on finishing of energization for spot welding, so that a spot welded portion (a nugget portion and a heat-affected zone) is annealed, and thereby a hardness of the welded portion is reduced. However, in these methods, there is a problem that productivity is reduced since welding in both methods requires a long period of time. Further, there is also a problem that a peel fracture in a nugget is apt to occur due to softening of the welded portion by tempering.
Methods for solving a problem of joint strength reduction are described in Patent Literatures 2 and 3. In these methods, tempering treatments are performed by heating welded portions with high frequencies after spot welding. However, in these methods, another process is necessary after welding, which is complicated, and there also occurs a problem that a special apparatus for using the high frequency is required and so on. Further, there is also a problem that a peel fracture in a nugget is apt to occur due to softening of a welded portion by tempering.
Non-patent Literature 2 and Patent Literature 4 describe that a tensile strength (a peel strength) of an L-shaped welded joint is improved by heating and temperature keeping by a paint baking treatment among automobile production processes. However, in neither Non-patent Literature 2 nor Patent Literature 4, a relation between a type of steel or a steel plate component (a carbon equivalent, in particular) and a cross tensile strength is not clarified. Further, detailed fracture appearances of welded portions nor relations between the fracture appearances and cross tensile strengths in a case that various steel types are used are not indicated.
In addition to the techniques above, it can be considered to perform spot welding with a steel plate with a low carbon equivalent as indicated in Non-patent Literature 3, for example. However, in this method, there occurs a problem that manufacturing of a steel plate itself becomes difficult or a necessary mechanical characteristic cannot be obtained or the like because of restriction of components. Further, in a case that a hot stamping steel plate on the order of 1470 MPa described in Patent Literature 5 is used, for example, a quenching treatment is necessary, and thus reducing a carbon equivalent is difficult.
Further, there is known a technique in which the number of spot welding is increased in spot welding a high-strength steel plate. However, in this method, there is a problem that a welding operation efficiency deteriorates thereby reducing a productivity. Further, there is a problem of increase of a welding execution cost, restriction of a degree of design freedom, and so on.
Patent Literature 6 describes a method in which spot welding is performed with a predetermined high-strength steel plate and a hardness of a welded portion is reduced by performing tempering energization under a predetermined condition after welding energization. As the predetermined high-strength steel plate, there is used one whose carbon equivalent is prescribed within a predetermined range and in which an incline of a stress-distortion diagram within a range of a true strain of 3 to 7% obtained in a tensile test is prescribed to be equal to or more than 5000 MPa. However, also in this method, there is a problem that a peel fracture in a nugget is apt to occur due to softening of a welded portion.
Patent Literature 7 describes a method in which post-heating energization is performed after nugget formation by main energization, at a current value of equal to or more than a current value of the main energization. Further, Patent Literature 8 describes a method in which post-heating energization is performed by increasing a pressurizing force after nugget formation by main energization. However, even by these methods, there is a problem that a high joint strength cannot be obtained stably.
It should be noted that as a method for preventing an occurrence of a defect or a crack in a nugget there is known a method in which a pressurizing force of a welding electrode against a high-strength steel plate is increased after welding. However, in this method, there is a problem that since quite a high pressurizing force is necessary a welding gun having a high stiffness is required, making the method unsuitable for a practical use. Further, there is also known a method in which a cooling speed after welding is reduced by performing post-heating energization subsequently soon after welding. In this method, since a contraction speed of a welded portion is reduced accompanying reduction of cooling speed, defect or crack is hard to occur. However, even this method is not necessarily effective in spot welding of a high-strength steel plate whose tensile strength is equal to or more than 750 MPa and whose carbon amount or carbon equivalent is high, which is used for a reinforcing component of an automobile and so on.